1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly relates to a liquid crystal display device that can display an image of quality with its power dissipation reduced by utilizing reflected light.
2. Description of the Related Art
As various types of portable electronic appliances, including cell phones and personal digital assistants (PDAs), have become more and more popularized, liquid crystal display devices, which are often built in these appliances, are increasingly required to reduce their power dissipation. Meanwhile, the amount of information to be displayed on the liquid crystal display devices has also been on the rise. Thus, the liquid crystal display devices also have to further improve the quality of an image to be displayed thereon.
To provide a liquid crystal display device that can display an image of quality with its power dissipation reduced, the present inventors carried out an intensive research on a method of driving a TFT liquid crystal display device of the reflection type at a decreased frequency. As a result of experiments, the present inventors discovered and confirmed that if the image on the display is refreshed at a decreased rate, then a flicker (or variation in brightness) is produced and cannot be eliminated even by adjusting the so-called “counter voltage shift”. Hereinafter, the relationship between the flicker and the counter voltage shift will be described.
In a TFT liquid crystal display device, a feedthrough phenomenon occurs in the voltage being applied to pixel electrodes due to the parasitic capacitance formed by its TFTs and the switching operations of the TFTs. Accordingly, to compensate for such a feedthrough voltage, an offset voltage, which has its amplitude defined in accordance with the feedthrough voltage, is applied to a counter electrode that is disposed so as to face the pixel electrodes by way of a liquid crystal layer.
However, if the feedthrough voltage is not equal to the offset voltage (the difference between the feedthrough and offset voltages is sometimes called a “counter voltage shift”), then the effective voltage to be applied to the liquid crystal layer changes every time the polarity of the voltage is inverted. As a result, the observer senses that voltage variation as a flicker.
Even for a normal liquid crystal display device to be driven at a refresh rate of 60 Hz, various countermeasures are taken to make such a flicker as insensible as possible. Examples of those countermeasures include a so-called “gate line inversion” (which is also called a “1H inversion”) technique, by which the polarity of the applied voltage is inverted on a gate line basis. However, the counter voltage shift might sometimes be too great to be eliminated by any of those countermeasures. In that case, the flicker might be sensed just like a moving striped pattern.
The present inventors carried out experiments on a reflective liquid crystal display device having pixel pitches of 60 μm×RGB×180 μm to find a counter voltage shift value at which no flicker was perceivable in a half-tone display state. Consequently, the present inventors discovered and confirmed that where the observer was watching the image on the display carefully, a counter voltage shift of about 250 mV resulted in a quite perceivable flicker even when the device was driven by the gate line inversion technique.
If the liquid crystal display device is driven at a decreased frequency to reduce its power dissipation, that flicker resulting from the counter voltage shift gets even more noticeable. For example, if the device is driven at 5 Hz, even a counter voltage shift of as small as 30 mV makes the line-by-line difference in brightness between the gate lines easily perceivable. What is worse, the refresh period (i.e., vertical scanning period) is as long as 200 ms. Accordingly, in that case, the observer can clearly see with his or her own eyes how bright and dark lines are alternated on a vertical scanning period basis. Thus, such a liquid crystal display device is far from being a commercially viable product.
That counter voltage shift of about 30 mV is so small as to be easily created due to any of a number of inevitably occurring variations that include: a variation in thickness of the liquid crystal layer during the manufacturing process; a small variation in temperature of the liquid crystal layer according to the operating environment; and degradation in electrical or physical properties of the liquid crystal material or alignment film material with time. Nevertheless, when a huge number of liquid crystal display devices should be produced, it is very difficult to reduce the counter voltage shift to less than 30 mV by adjusting the offset voltage to be applied to the counter electrode. A counter voltage shift that can be compensated for by the currently available technique is at least about 100 mV.
The present inventors discovered and confirmed via experiments that when the refresh rate is about 45 Hz or less, the flicker is too much noticeable to be eliminated by any of the currently available counter voltage shift adjustment techniques.
The results of our experiments also revealed that the flicker is perceivable particularly easily in a reflective/transmissive liquid crystal display device (which will be herein referred to as a “dual-mode liquid crystal display device”) in which each pixel thereof includes a reflective portion for conducting a display operation in a reflection mode and a transmissive portion for conducting a display operation in a transmission mode. In the dual-mode liquid crystal display device, the flicker also becomes particularly noticeable when the refresh rate is as low as about 45 Hz or less. However, in the device of this type, the flicker is perceivable even more easily than a reflective or transmissive device. Accordingly, some countermeasure must always be taken for the dual-mode device, not just when the device is driven at a decreased frequency.